(a) Field of the Invention The present invention relates to a method of protecting cells against damage caused at least in part by apoptosis, comprising administering to subjects a therapeutic dose of leumorphin having cytoprotective activity, and a pharmaceutical composition comprising an effective amount of leumorphin having a cytoprotective activity.
(b) Description of the Related Art
Endogenous opioid peptides, found in the central and peripheral nervous systems, play important roles in a wide variety of physiological and pathophysiological conditions. Multiple peptides are derived from their precursors, proopiomelanocortin, proenkephalin and prodynorphin, through hydrolysis by proteases.
Prodynorphin undergoes endoproteolysis at specific sites, which results in the generation of leumorphin, neoendorphin, rimorphin (dynorphin B) and dynorphin (dynorphin A) (Waldhoer et al., 2004). Dynorphin A, the most abundant form of dynorphins, has been shown to be involved in a range of functions, and activation of the κ-opioid receptor (KOR) accounts for many of its biological activities (Solbrig and Koob, 2004).
The pharmacological and functional properties of other prodynorphin gene products, however, have been characterized to a much lesser extent. It has been reported that leumorphin can be further processed and converted to rimorphin by endopeptidases (Berman et al., 1995), but leumorphin and rimorphin have comparable affinities for the KOR, implying that further processing may not be necessary to achieve maximal affinity and activity (Mansour et al., 1995). Although there are multiple products processed from a single gene, functional diversity of the peptides has not been clearly elucidated (Suda et al., 1983a; Mansour et al., 1995).
Increasing pharmacological and biochemical evidence suggests that various responses mediated by the opioid peptides cannot entirely be explained by activation of opioid receptors (Wollemann and Benyhe, 2004). Peptides exhibiting the so-called “non-opioid” effects include β-endorphin (Navolotskaya et al., 2002), dynorphin (Walker et al., 1982), nociceptin (Mollereau et al., 1996) and Met-enkephalin (Zagon et al., 1991). Classical pharmacological definition of opioid actions involves the antagonism of their effects by naloxone, a general antagonist of the opioid receptors, whereas the non-opioid actions are insensitive to naloxone. Existence of these non-opioid actions might provide an explanation for the diverse and complex pharmacological and physiological properties elicited by opioid peptides in vivo.
Opioids produce strong analgesic effects in animals. In addition to these well-recognized effects, increasing evidence suggest that opioids elicit a variety of biological responses that appear to be independent of their analgesic properties, but may rather have effects on cell survival and proliferation (Dermitzaki et al., 2000; Tegeder and Geisslinger, 2004). These effects have been suggested to involve activation of protein kinase B (AKT) and/or extracellular signal-related protein kinase (ERK) signaling pathways. Some of the observed effects have been suggested to be downstream of opioid receptors (Polakiewicz et al., 1998; Persson et al., 2003), whereas others have been shown to be insensitive to opioid receptor antagonists or pertussis toxin (Moon, 1988; Tegeder and Geisslinger, 2004), implying that some of the effects might be independent of opioid receptor activation. It is still unclear whether these responses are mediated through activation of typical opioid receptors and inhibitory G-protein-signaling.
Although many distinct endogenous opioid peptides have been identified, it has not been clearly demonstrated so far whether the products of prodynorphin exhibit functional and pharmacological diversity. In order to address this issue, we examined the effects of endogenous opioid peptides derived from prodynorphin on intracellular signaling events and cell viability in rat pheochromocytoma PC12 cells that express and secret prodynorphin as well as its cleaved products (Margioris et al., 1992).